stmhal: Adjust computation of SYSCLK to retain precision.

Author: dpgeorge

stmhal/hal/src/stm32f4xx_hal_rcc.c

@@ -972,7 +972,12 @@ uint32_t HAL_RCC_GetSysClockFreq(void)
  if (__RCC_PLLSRC() != 0)
  {
  /* HSE used as PLL clock source */
- pllvco = ((HSE_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> POSITION_VAL(RCC_PLLCFGR_PLLN)));
+ //pllvco = ((HSE_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> POSITION_VAL(RCC_PLLCFGR_PLLN)));
+ // dpgeorge: Adjust the way the arithmetic is done so it retains
+ // precision for the case that pllm doesn't evenly divide HSE_VALUE.
+ // Must be sure not to overflow, so divide by 4 first. HSE_VALUE
+ // should be a multiple of 4 (being a multiple of 100 is enough).
+ pllvco = ((HSE_VALUE / 4) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> POSITION_VAL(RCC_PLLCFGR_PLLN))) / pllm * 4;
  }
  else
  {